The present invention relates to a fluid flow sensor for measuring a fluid flow rate by using a heating resistor and, for example, it relates to a fluid flow measurement device suitable to a measurement for an intake airflow rate of an internal combustion engine.
Of various types of the fluid flow sensors that are, for example, disposed in various intake air passages of internal combustion engines in automobile cars etc., a thermal type fluid flow sensor has come into widespread use because of their capability to directly sense mass air flow rate.
In such thermal type fluid flow sensors, especially, those by using a sensor elements manufactured by semiconductor micromachining technology, which are provided on the semiconductors substrate such as silicon (Si), are advantageous in terms of economical mass-producibility, and low-power driving with size reduction. Therefore, thermal type fluid sensor using a sensor element based on this semiconductor technology has gained the spotlight in recent years. Such fluid flow sensors are described in Japanese Patent Laid-Open No. 2002-48616 (Patent Document 1).
In the fluid flow sensor described in the Patent Document 1, sensing resistors as sensing elements are formed on a silicon substrate by interposing an insulating layer between the heating resistor and the thin film. In such a manufacturing process, a portion of the silicon substrate is removed to form a thin film (diaphragm portion) for thermally insulating the resistance. A heating resistor is formed on the thin film to be driven as a heater. Plural thermal sensitive resistors for measuring fluid flow is disposed on the thin film of the substrate so as to be located on an adjacent upstream side and an adjacent downstream side of the heating resistor in a stream direction of fluid to be measured. Fluid flow rate is sensed by a measuring difference of temperature between the upstream side thermal sensitive resistor and the downstream side thermal sensitive resistor. In the measuring method based on difference of temperature, the heating resistor is heated at a constant temperature and heating the thermal sensitive resistors at the upstream and at the downstream by heat conduction and heat transfer. In a case where the air flow rate is not present, the upstream side thermal sensitive resistor and the downstream side thermal sensitive resistor are theoretically heated identically, and the temperature difference therebetween is substantially zero. When the air flow rate is present, since the upstream side thermal sensitive resistance is cooled and the temperature is lowered, but the downstream side thermal sensitive resistance is little cooled because heated air flows through the down stream side resistance. Accordingly a temperature difference occurs between them. Since the temperature difference corresponds the air flow rate, the air flow rate can be sensed based on the amount of the temperature difference. Since the upstream side and downstream side thermal sensitive resistors change their resistance values in accordance with the respective temperatures, voltage signals in accordance with the flow rate can be obtained by utilizing the change of the resistance values.
As shown in FIG. 1 and FIG. 6 of the Patent Document 1, by forming thermal sensitive resistors at the upstream and the downstream each by two pairs to form a bridge circuit, the sensor sensitivity can be doubled. Further, the measuring method based on the difference of temperature can detect the flowing direction of air. In a case where the air flow rate occurs in the direction opposite to that described above, the thermal sensitive resistor at the downstream is cooled. Accordingly, the circuit constitution shown in FIG. 6 of Patent Document 1 generates an output in the direction opposite to the point of zero flow rate. By the provision of the direction detecting means, the air flow rate can be measured more accurately than the fluid flow sensor not having the direction detecting means in a running state of causing an air flow in the directing from an engine to an air cleaner (reverse flow). In the prior art, pulsation of intake air increases at a low speed of 4-cylinder engines to often cause reverse flow near the full open state of a throttle. However, pulsation and reverse flow tend to occur at high speed to increase amount of the reverse flow in engine adopted for complicate control such as change of vale on-off time for coping with exhaust gas regulation and requirement for reducing fuel cost in recent years. Further, pulsative flow including reverse flow occurs also in a four or more multi-cylinder engine. Accordingly, the direction detecting function is an extremely effective means.
Further, thermal response at high speed to flow rate change can be obtained by forming such flow-rate measurement resisters on the thin film as diaphragm. In a case where a high-response fluid flow sensor is applied to the control of an automobile, it can response to the abrupt change of flow rate, or to the occurrence of pulsation in an air intake pipe. Accordingly, it is possible to measure the air flow rate more accurately than the fluid flow sensor of large heat capacity and at slow response speed.
The fluid flow sensor described above comprises mainly a flow sensing element, a basis for mounting the flow sensing element, a circuit for driving the flow sensing element and a case for mounting components described above and attached to an intake pipe for flowing intake air, and the flow sensing element is disposed in a bypass passage as a secondary passage.